This invention relates to electronic article surveillance systems using magnetic phenomena and, in particular, to markers, methods and apparatus for use in such article surveillance systems.
Electronic article surveillance systems in which magnetic markers are used to detect the presence of articles under surveillance are well known in the art. French patent No. 763,681 to Picard discloses an early system of this type. The Picard patent teaches that low coercive force, high permeability metals, such as permalloy, when subjected to an alternating magnetic field induce harmonics which distinguish these metals from other magnetic metals. These metals with their unique harmonics can thus be used as magnetic markers to identify objects which carry the markers.
Since the early days of the Picard patent, substantial effort has been expended in an attempt to improve the existing markers. This effort, for the most part, has been directed at finding new materials having a lower coercive force and higher permeability than was previously used. Because the voltage pulse generated by the presence of the marker is dependent on the hysteresis characteristic of the magnetic material of the marker, by using materials with lower coercive force and higher permeability, higher order harmonics with higher amplitude values could be realized for lower values of applied field, thereby making the markers more distinguishable.
While the search for materials with higher permeability and lower coercive force was thus the direction of most researchers, a radically different approach is presented in U.S. Pat. No. 4,660,025, entitled "Article Surveillance Magnetic Marker Having an Hysteresis Loop With Large Barkhausen Discontinuities", and assigned to the same assignee hereof. In the '025 patent, a magnetic marker is disclosed which does not depend upon a high permeability, low coercive force material. Furthermore, the output pulse developed in response to the presence of the marker is substantially independent of the time rate of change of the interrogating field and the field strength as long as the field strength exceeds a low minimum threshold value. More particularly, the '025 patent teaches that by forming the marker so that the magnetic material of the marker retains stress, the marker exhibits a hysteresis characteristic having a large Barkhausen discontinuity. Accordingly, upon exposure to an interrogating field exceeding the low threshold value, the magnetic polarization of the marker undergoes a regenerative reversal. This so-called "snap action" reversal in the magnetic polarization results in the generation of a sharp voltage pulse, rich in high harmonics, which affords a more distinguishable detectable signal.
In addition to the highly advantageous harmonic content and pulse output of the marker of the '025 patent, the marker is also advantageous in that it allows for deactivation by a number of techniques. These techniques are disclosed in U.S. Pat. No. 4,686,516, entitled "Method, System and Apparatus for Article Surveillance", and also assigned to the same assignee hereof. More particularly, the '516 patent discloses one practice for deactivating the marker of the '025 patent in which the amorphous material of the marker is crystallized. This is accomplished by heating at least a portion of the marker above the crystallization temperature, either by application of an electric current or radiant energy such as laser light. Another procedure disclosed in the '516 patent and useable with this type of marker involves the application of mechanical or radiant energy means to relieve the internal stress in the marker. While some of these deactivation procedures enable deactivation without touching the marker, they also require careful application of the deactivation energy so that the energy is not blocked from reaching adjacent articles.
It is therefore a primary object of the present invention to provide an improved magnetic marker for electronic article surveillance systems wherein the marker undergoes snap action or step changes in its magnetic flux at low threshold values of the applied field, while also being hands-off (i.e., non-contact) deactivatable by simple means.
It is a further object of the present invention to provide a method of making the aforementioned improved magnetic marker.
It is still a further object of the present invention to provide an electronic article surveillance system incorporating the aforementioned improved magnetic marker.
It is yet a further object of the present invention to provide an electronic article surveillance system incorporating both deactivation means and the aforementioned improved magnetic marker.